It is well known in the art that the resolution of apparatus responding to electromagnetic radiation in the optical (or near optical) regions, can be improved by increasing the aperture. Likewise, the signal-noise ratio of the apparatus can also be improved by increasing the aperture. Furthermore, if the apparatus is to detect targets against an irrelevant background, then the ratio of target signal to background clutter may be improved by improving the resolution.
However, an increase in the size of the aperture results in an increase in the size and weight of the instrument with corresponding disadvantages; particularly the difficulty of mounting and transporting the apparatus. For example, if the apparatus is to be installed in a vehicle such as a space satellite, a missile or an aircraft where space is limited and where each kilogram of payload requires many kilograms of vehicle and propulsion, then size and weight are critical attributes having a great effect on the cost and practicality of the overall system. In some particular applications, no vehicle may be available which can accommodate the size of the aperture required for a particular mission. In such a case, the practicality and cost of the overall mission may be driven by the sensor requirement through development of a special vehicle or the use of extraordinary deployment techniques. In earth-bound applications, similar considerations apply. For example, in an astronomical application it may be desirable to limit the size of the apparatus which is traversed.
It is also well known in the art that the signal to noise ratio of optical instruments which must gather data on an image within a limited period of time can be improved by increasing the number of radiation detectors used to transduce the electromagnetic radiation to electrical signals. However, an increase in the number of such detectors entails similar disadvantages.
For reasons such as are briefly set out above, therefore, it has long been a goal in this art to obtain the ultimate performance from these instruments. The prior art evidences the use of a technique referred to as synthetic aperture which is believed to be useful in attaining high performance from instruments of this sort. In this regard, see the Meinel et al. article entitled "A Large Multiple Mirror Project", appearing in Optical Engineering, Volume 11, Number 2 (March/April 1972), pages 33-37, and the article by Meinel entitled "Aperture Synthesis Using Independent Telescopes", appearing in Applied Optics, Volume 9, Number 11 (November 1970), pages 2501-2504. In the latter article, the author defines aperture synthesis as "occurring when separate optical elements are combined with phasing to form a common image field in which the resolution is greater than that for a single element".
In 1890, A. A. Michelsen (Phil. Mag. (5), 30, 1) utilized two auxiliary mirrors to augment the effective diameter of the 100 inch telescope at Mount Wilson to increase its resolution for measuring the diameter of celestial objects. The two mirrors were moved laterally until the contrast of fringes in the pattern of interference of light from the two apertures was extinguished. The separation of the mirrors was then a measure of the diameter of the object.
J. S. Wilczynski, in U.S. Pat. No. 3,556,630, discloses a "Method and Apparatus for Obtaining, by a Series of Samples, the Intensity Distribution Across Sources of Incoherent Electromagnetic Waves to Produce a Single Composite Picture". He discloses how a desired "large" aperture area can be subdivided into a plurality of smaller apertures of equal size. The image that would be obtained by the "large" aperture is then derived using only a pair of the smaller apertures by physically relocating the pair of small apertures to a number of different positions.
However, due to the necessity for mechanically relocating the plurality of mirrors to a precision comparable to the wavelength of the radiation sensed, which requires moving substantial masses and which takes time, during which time the image must be fixed or substantially fixed, applications for this technique are limited.
From the foregoing it should be apparent that there is still a need to increase the ability to extract information from a given sized aperture or from an aperture of a given configuration and from a given array of detectors.
In recent work (see J. A. Jamieson, "Passive Infrared Sensors: Limitations on Performance", Applied Optics, Volume 5, page 891, April 1976), I have shown that a passive sensor gathers information in four dimensions of time, wavelength, and two angles, that frequently, information gathered in one dimension is not required to fulfill the mission of the sensor, but that the information acquired in another dimension is inadequate. A principal purpose of this invention is to allow information gathered in wavelength to be utilized to enhance information gathered in angle. That is, it is an object of the invention to use data measured by the apparatus which characterize the spectral distribution of radiance of a radiating scene to augment data measured on the scene in angle or location in the object surface. Another object of the invention is to provide apparatus which can be used adaptively for spectral resolution or spatial resolution beyond that ordinarily available or any intermediate combination of spectral and spatial resolution without the need to reconfigure the apparatus.
Another principal object of the invention is to provide apparatus which can yield spatial resolution beyond that ordinarily available from a given size of aperture. Another principal object is to provide apparatus which can measure an image of an object scene at a resolution beyond that ordinarily available from a given number of detectors. Another object is to provide apparatus in which the spatial response can be adjusted readily without the need to reconfigure the apparatus (e.g., to emphasize high spatial frequencies for edge sharpening or alternatively in the same instrument at another time to treat all spatial frequencies equally for best fidelity of response). Another object of the invention is to provide apparatus in which failure or degradation in performance of some of the detectors causes a minimum impact on the capability of the apparatus to yield a complete image.
A particular advantage of the invention is that the entrance pupil is incompletely filled so that other apparatus required for an overall mission in which this apparatus is used can be colocated with the entrance pupil or alternatively the entrance pupil can be distributed (e.g., on either side of the nose of a missile or on the wings of an aircraft) so as not to interfere with other functions of the overall system. Another advantage is that the radiation detectors in the apparatus are not required to fill the focal plane so that space remaining can be utilized for other functions such as electrical connections, pathways to remove heat, to minimize crosstalk, or for preamplifiers, charge-coupled devices, and other auxiliary electronic apparatus, or for redundant detectors.
Another advantage of the invention is that it provides an improved capability to discriminate a target object from a structured radiant background. This advantage results initially from the improved spatial resolution of the apparatus but is further enhanced if the target object has a different spectral distribution of radiance than the structured background.
A further advantage is that the invention reduces the need for accurate a-priori knowledge of target and background signatures before a system using a sensor is designed or committed to its mission. This advantage derives from the adaptive nature of the spatial/spectral response of the invention. This attribute allows the spectral response to be modified by reprogramming the data processing without reconfiguring the apparatus after the system is committed to its mission. If a mission should require the sensor to perform well against several kinds of targets at different times, the response can be successively altered an unlimited number of times rather than adopting an inferior fixed compromise response.
A further advantage is that the system designer can select the maximum dimensions of the entrance pupil independently of the collecting area of the entrance pupil so that he may select resolution and sensitivity independently to achieve a balanced, economical design.